I basically treat it as allowing a person to move at their normal speed across intervening gaps, allowing use of Athletics or Acrobatics to go up a speed level much like "sprinting". Going up, or using it to lower yourself, is an interesting proposition, but one I feel usually comes down to buying other other Movement powers for your grapnel gun. For example, for one game, I had the following:Grappling Hook [Super-Movement 4, slow fall, swinging, wall-crawling 2 (full speed)] - For me, that let me do everything from swinging from convenient support beams to rapidly zipping up the side of a wall to catching myself while falling.

I got a clarification for what the player wants, he plays the game DC Universe Online and he wants to mimic the grappling the Acrobatic movement set has, namely the wall pull and upward grapple pull (Which is faster than your wall crawl.)

I like the Leaping, but I'm thinking it would have 'Platform' or 'Limited to reachable surfaces', cuz it doesn't allow leap if there's nothing for the grapnel to cling to.

I like the Acrobatics skill idea because they're trying to play a Robin type (I may end up the Robin, as we're trying a Hero and Sidekick short campaign) but at the same time, a bad roll and splat. Slow Fall sounds like a good idea to help mitigate that, and lower the cost.

Swinging to me is basically flight limited to any place you can grapple on, and has a glide mechanic in that the hero must end on a surface at some point. SO as long as there are reasonable grapple locations the hero can go up down forward backwards and in any reasonable direction they want.

the requirement of the occasional athletics or acrobatics checks as Fuzzy suggests for increasing speed or performing some sort of advanced maneuvers is a great idea and one I use as well.

The use of extra effort is where you get the neat little tricks such as using it as a grapple or extra limb etc etc.

and as Ken and Fuzzy suggest one could always expand the scope of the grapple device via alternate powers and power frameworks.

Dr. Silverback has wryly observed that this is like trying to teach lolcats about Shakespeare